Many electrical devices are incorporating touchscreen type displays. A touchscreen is a display that detects the presence, location, and pressure of a touch within the display area, generally by a finger, hand, stylus, or other pointing device. The touchscreen enables a user to interact with the display panel directly without requiring any intermediate device, rather than indirectly with a mouse or touchpad. Touchscreens can be implemented in computers or as terminals to access networks. Touchscreens are commonly found in point-of-sale systems, automated teller machines (ATMs), mobile phones, personal digital assistants (PDAs), portable game consoles, satellite navigation devices, and information appliances.
There are a number of types of touchscreen technologies. A capacitive touchscreen panel is coated, partially coated, or patterned with a material that conducts a continuous electrical current across one or more capacitive touch sensors, referred to simply as touch sensors. The touch sensors exhibit a precisely controlled field of stored electrons in both the horizontal and vertical axes to achieve capacitance. The human body is also an electrical device that has stored electrons and therefore also exhibits capacitance. When a reference capacitance of the touch sensors is altered by another capacitance field, such as a finger, electronic circuits located at each corner of the panel measure the resultant distortion in the reference capacitance. The measured information related to the touch event is sent to a capacitive touchscreen controller for mathematical processing. Touch sensors can either be touched with a bare finger or with a conductive device being held by a bare hand. Touch sensors also work based on proximity, and do not have to be directly touched to be triggered. In most cases, direct contact to a conductive metal surface does not occur and the touch sensors are separated from the user's body by an insulating glass or plastic layer. Devices with capacitive buttons intended to be touched by a finger can often be triggered by quickly waving the palm of the hand close to the surface without touching.
A capacitive touchscreen is made of an array of touch sensors arranged into rows and columns. A channel can refer to either a single sensor, a row sensor, or a column sensor. In a typical capacitive touchscreen application, the capacitance of each channel is sequentially measured to generate a profile of capacitance change in the capacitive touchscreen. This capacitance profile can be used to detect the presence of a touch event and to report the position of the touch coordinates. Typically, the sampling for each channel is done at a fixed interval. However, this can form beat frequencies that are the same as certain noise frequencies.
Finger-coupled noise is the noise coupled through a finger or a conductive stylus to one or more touch sensors when the user is touching a capacitive touchscreen. Only the touch sensors beneath the finger are affected by the finger-coupled noise. The human body acts an antenna which may pick up ambient noise, such as noise generated by surrounding compact flourescent lights, which is subsequently passed to the touch sensors when touched. The human body is also grounded to the earth, which can be a different ground than the device ground of the capacitive touchscreen. With two different grounds, ground noise will also be added to the overall system. The finger-coupled noise can be of any frequency ranging from close to DC to hundred's of kilohertz (kHz) with an amplitude up to a few volts peak-to-peak. When the noise level at a particular frequency is high enough, it can cause the capacitive touchscreen controller to report a touch when no touch is actually present, referred to as a false touch.